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The double-polarization observable $E$ and the helicity-dependent crosssections $\sigma_{1/2}$ and $\sigma_{3/2}$ have been measured for the firsttime for single $\pi^{0}$ photoproduction from protons and neutrons bound inthe deuteron at the electron accelerator facility MAMI in Mainz, Germany. Theexperiment used a circularly polarized photon beam and a longitudinallypolarized deuterated butanol target. The reaction products, recoil nucleons anddecay photons from the $\pi^0$ meson were detected with the Crystal Ball andTAPS electromagnetic calorimeters. Effects from nuclear Fermi motion wereremoved by a kinematic reconstruction of the $\pi^{0}N$ final state. Acomparison to data measured with a free proton target showed that the absolutescale of the cross sections is significantly modified by nuclear final-stateinteraction (FSI) effects. However, there is no significant effect on theasymmetry $E$ since the $\sigma_{1/2}$ and $\sigma_{3/2}$ components appear tobe influenced in a similar way. Thus, the best approximation of the twohelicity-dependent cross sections for the free neutron is obtained by combiningthe asymmetry $E$ measured with quasi-free neutrons and the unpolarized crosssection corrected for FSI effects under the assumption that the FSI effects aresimilar for neutrons and protons.

physics letters b

First measurement of the polarization observable $E$ and  helicity-dependent cross sections in single $\pi^{0}$ photoproduction from  quasi-free nucleons